The invention relates generally to control systems which require loadfeel and/or backdrive and, in particular, to such systems requiring accuracy of the loadfeel breakout force and backdrive.
The value of providing tactile feedback to the controls of an aircraft have long been recognized, and many systems providing such tactile feel have been designed. Typically, it is desirable that the tactile feel or resistance forces applied to the controls be proportional to the G-forces resulting from operation of the controls. This provides improvement in handling characteristics of the aircraft and reduces the possibility of overstressing the aircraft during high speed maneuvers. In aircraft which also include automatic pilot systems, it is generally desirable to have a system which positions the controls in accordance with the positioning of the control surfaces of the aircraft i.e., a system which provides backdrive.
Some prior art manual control feel systems have been designed to be responsive to sensor inputs relating to the forces acting on the control surfaces. Examples of such prior art systems are disclosed in U.S. Pat. No. 4,236,685 to Kissel and U.K. Patent No. 2,061,843 to Smith. The Kissel system provides a manual control feel system in which the degree of feel provided is based on data pertaining to static pressure and other information from the controlled surfaces. The Smith system includes an hydraulic actuator which provides a feel force to the control lever in response to the aircraft position as well as in response to pitot and static pressure differential forces on the control surfaces. However, a primary disadvantage of such systems is that they do not provide data input pertaining to frictional forces of the control linkage and other mechanical subsystems. Since such forces may be of significant magnitude, their omission detracts from the precise feel such a control system would otherwise have. In addition, since such systems also do not include input pertaining to the pilot's manual forces exerted on the control lever, the tactile feedback such systems provide is not as precise or sensitive as may be desirable in some applications.
Many other prior art manual control systems for aircraft include one or more springs which resist movement of the control stick in all directions at all times during operation of the stick. An example of such a manual control system is disclosed in U.S. Pat. No. 4,477,043 to Repperger. The Repperger system includes a spring which is connected at its midpoint to the aircraft control stick and an actuator connected to opposite ends of the spring for compression thereof. The spring is utilized to resist movement of the stick from a neutral position. In response to information pertaining to movement of the aircraft, the actuators compress the spring thereby increasing its resistance to movement of the stick. However, as with the Smith system, the Repperger system does not provide continuous monitoring of the degree of manual force applied by the pilot to the stick at all times during operation thereof. Consequently, the precise feel provided by the Repperger system is compromised by this deficiency. Thus, the Repperger system has the disadvantage that it cannot be utilized in many aircraft applications because it is specifically designed for and thus limited to pure fly-by-wire systems in which there is no direct mechanical linkage between the pilot operated control member and the aircraft control surfaces.
Other prior art systems for providing feel to a manual control utilize a computer for determining the resistive forces applied to the control. An example of such a prior art system is disclosed in U.S. Pat. No. 4,516,063 to Kaye. The Kaye system utilizes a stepper motor connected to a computer to generate detents and feel forces resisting movement of the control member. The detent positions may be varied according to inputs relating to altitude of the aircraft, etc. However, as with the Repperger and Smith systems, there is no input to the computer regarding the forces applied by the operator to the manual control. Consequently, systems such as the Kaye system do not have a high degree of precision in control of the loadfeel.
Other control feel systems utilize a combination of devices to bias the control. An example of such a prior art system is disclosed in U.S. Pat. No. 4,580,210 to Nordstrom. The Nordstrom system utilizes inputs relating to current as well as previous positions of the control which are fed to a computer which provides a biasing force. The biasing force is directly provided by a spring and a torque motor. However, the Nordstrom system does not utilize any information inputs from the controlled motor or the aircraft outer surface structures (relating to aerodynamic forces acting thereon or flight conditions). Thus, not only does the Nordstrom system lack a sensor for determining the manual forces applied to the control but it also fails to utilize information regarding the controlled surface aerodynamic forces. Consequently, the Nordstrom system is not as sensitive or precise as may be desired in some applications.
The prior art manual control feel systems described hereinabove typically simply utilize the position of the manual control lever to determine the resistive force provided to the control lever. However, U.S. Pat. No. 5,076,517 to Ferranti discloses a system which also indirectly incorporates data relating to the manual forces applied to the control lever by utilizing an input relating to the force output of the control stick. The Ferranti system measures the force output of the control stick which is purportedly proportional to the degree of force exerted on the stick by the operator. Data relating to this control stick force is utilized to provide a signal to a motor to position the stick in response to that data. Although damping of the control stick is provided, application of resistive force to the stick is not provided in accordance with data relating to the pilot's manual forces applied to the stick. Consequently, this prior art system does not have a manual control stick force feedback loop utilized to provide a sensitive control stick feel based on desired parameters of aerodynamic forces on the controlled surfaces, flight conditions, etc.
What is thus needed is a control system having loadfeel and backdrive in which the loadfeel utilizes a control feedback loop to adjust the resistive forces applied to the control stick in response to the operator's forces exerted thereon. What is also needed is a control system with loadfeel and backdrive in which the system provides loadfeel by utilizing information relating to frictional forces acting on the mechanical back-up subsystem of the control system and/or frictional forces acting on the hydraulic and uther mechanical components of a fly-by-wire control system.